For transport by ferry (via a ferry terminal) it is possible to request the status for a given shipment ID (which is why you are asked for a shipment number). For the connected shortsea and other terminals, the information can be requested for a specific container (which is why you are asked for a container number).
Shipment numbers of ferry-operators have the following format (SCAC + bookingnumber + sequence number):
CLDN: CLVW0012345678001
DFDS/KVSA: DFDS123456780001
P&O: PONFHU12345678001
Stena Line: STNL0012345678001